1. Field of the Invention
Embodiments of the present invention relate generally to optical communication systems and components and, more particularly, to a liquid crystal-based optical switch and attenuator.
2. Description of the Related Art
In optical communication systems, it is sometimes necessary to perform 1×2 switching of an optical signal, where an input light beam enters an optical switching device through an input port and is directed to one of two output ports. There are also more complicated optical switching schemes, such as 2×2, 1×N, and N×N optical switches, which may be realized by combining multiple 1×2 optical switches.
In addition to routing of signals by optical switches, attenuation of signals in optical communication systems is needed, for example in an optical communication system that employs wavelength division multiplexing (WDM). In such an optical system, information is carried by multiple channels, each channel having a unique wavelength. WDM allows transmission of data from different sources over the same fiber optic link simultaneously, since each data source is assigned a dedicated channel. The result is an optical communication link with an aggregate bandwidth that increases with the number of wavelengths, or channels, incorporated into the WDM signal. In this way, WDM technology maximizes the use of an available fiber optic infrastructure, such that what would normally require multiple optic links or fibers instead requires only one. In practice, different wavelength channels of a WDM signal typically undergo asymmetrical losses as they travel through an optical communication system, resulting in unequal intensities for each channel. Because these unequal intensities can compromise the integrity of the information carried by the WDM signal, an optical device or array of optical devices is used in WDM systems to perform wavelength-independent attenuation to equalize the respective intensities of the channels contained in a WDM signal.
Liquid crystal (LC) based optical switches are known in the art for switching and attenuation of the channels contained in a WDM signal, and in some applications offer significant advantages over other optical switch designs, but there is one drawback related to the polarization state of an input light beam. Because LC-based optical switches rely on rotating the polarization state of linearly polarized input light to perform switching functions, the input light beam must have a single known polarization state for such an optical switch to vary the optical path of the light beam as desired. However, optical signals transmitted over optical fibers are usually randomly polarized, i.e., the optical signals have a random superposition of the s- and p-components, and each polarization component must be treated separately by an optical switch.
One approach known in the art for managing s- and p-polarized components of a light beam for LC switching of the light beam involves performing a polarization “walk-off” with a birefringent optical element to spatially divide the light beam into s- and p-polarized light beams or components. Polarization walk-off can be performed when an optical signal is first introduced into an LC-based optical switch, for example, as the optical signal exits an optical input fiber and becomes a free-space beam. After a birefringent optical element separates the optical signal into two physically displaced s- and p-polarized components, the polarization of one of the components can be rotated 90° to match the polarization of the other. In this way, the optical signal is converted into a pair of closely spaced, parallel beams having the same polarization state, and this pair of beams can be treated together by the optical switch as a single light beam having a known polarization state. However, such an approach requires the optical signal to be in the form of two parallel beams, sometimes over a long path length, which increases the likelihood of large polarization dependent losses (PDL) that degrade signal quality. In addition, because a relatively large optical assembly is needed to perform the polarization walk-off as the optical signal exits the fiber, an undesirably large spacing between the input and output ports of the optical switch, e.g., greater than 1 mm, results.
Alternatively, an LC-based optical switch can divide an input beam into s- and p-polarized components, then manage the attenuation and switching of each component separately. Such an approach can result in significant PDL, however, due to the different attenuation performance of an LC material toward s- and p-polarized light. FIG. 1 illustrates the electro-optic behavior of an LC optical attenuator with respect to incident s- and p-polarized light and includes attenuation curves 191 and 192. The abscissa of graph 100 represents voltage applied to the LC optical attenuator and the ordinate of graph 100 represents resultant attenuation of a light beam normally incident on and passing through the optical attenuator. Attenuation curve 191 illustrates the attenuation of the light beam that is s-polarized and attenuation curve 192 illustrates the attenuation of the light beam that is p-polarized. As shown, the attenuation curve of p-polarized light differs substantially from the attenuation of s-polarized light. Thus, when the s- and p-polarized components of a light beam are both conditioned by the LC optical attenuator, each component is attenuated by a different amount, resulting in PDL 193. For example, at applied voltage Vo, the LC optical attenuator attenuates s-polarized light 10 dB and p-polarized light 12 dB, producing a PDL 193 of 2 dB.
While switching and attenuation of optical signals are known in the art, each of these operations is typically performed by a different optical device. The use of one optical device to perform switching and another device to perform attenuation in an optical communication system increases the size and complexity of the system, makes erosion of signal quality more likely due to misalignment of the optical devices, and requires a first independent control signal to complete the switching function and a second independent control signal to complete the attenuation function.
Accordingly, there is a need in the art for an optical switch for use in an optical network that has a minimal number of components and closely spaced input and output ports and can perform switching and low-PDL attenuation of an optical signal having an arbitrary combination of s-polarized and p-polarized light.